1. Field of the Invention
The present invention relates to a semiconductor device constructed by a semiconductor film that has a crystal structure, and more specifically, to a semiconductor device using a thin film transistor whose active layer is formed of a crystalline semiconductor film obtained through crystal growth on an insulating surface. The present invention also relates to a semiconductor device product ion system using laser light.
2. Description of the Related Art
In recent years, techniques for forming TFTs on a substrate have made great advancements and applications of TFTs to active matrix type semiconductor display devices are being developed. In particular, TFTs formed of polycrystalline semiconductor films (hereinafter referred to as polysilicon TFT) have higher field effect mobility (also referred to as mobility) than conventional TFTs that use amorphous semiconductor films, and accordingly can operate at high speed. Therefore pixels can be controlled by a driving circuit formed on the same substrate on which the pixels are formed, instead of a driving circuit external to the substrate as with the conventional technique.
Incidentally, for substrates used in semiconductor devices, a glass substrate is deemed more promising than a single crystal silicon substrate cost-wise. Glass substrates have poor heat-resistance and are easily deformed by heat. Therefore, when forming a polysilicon TFT on a glass substrate, using laser annealing to crystallize a semiconductor film in order to avoid thermal deformation of the glass substrate is extremely effective.
Laser annealing has characteristics such as remarkable reduction of processing time compared to an annealing method utilizing radiant heating or thermal conductive heating, and a semiconductor or a semiconductor film is selectively and locally heated so that a substrate is scarcely thermally damaged.
Note that the term “laser annealing” herein indicates a technique for recrystallizing a damaged layer formed on a semiconductor substrate or in a semiconductor film and a technique for crystallizing a semiconductor film formed on a substrate. The term “laser annealing” also includes a technique that is applied to leveling or improvement of a surface quality of the semiconductor substrate or the semiconductor film. Applicable laser oscillation devices are gas laser oscillation devices represented by an excimer laser, and solid laser oscillation devices represented by a YAG laser. Such laser oscillation devices are known to heat a surface layer of a semiconductor by laser beam irradiation for an extremely short period of time, i.e., about several tens of nanoseconds to several tens of microseconds so as to crystallize the surface layer.
Lasers are roughly divided into two types, pulse oscillation and continuous wave, by their oscillation methods. Pulse oscillation lasers are relatively high in output energy and therefore the size of laser beam can be set to several cm2 to increase the mass-productivity. In particular, if the shape of laser beam is processed by an optical system into a linear shape 10 cm or more in length, a substrate can be irradiated with the laser light efficiently to increase the mass-productivity even more. Accordingly, using pulse oscillation lasers to crystallize semiconductor films have been becoming mainstream.
In recent years, however, it has been found that the grain size of crystals formed in a semiconductor film is larger when a continuous wave laser is used to crystallize a semiconductor film than when a pulse oscillation laser is used. With crystals of larger grain size in a semiconductor film, the mobility of TFTs formed from this semiconductor film is increased while fluctuation in characteristics between the TFTs due to grain boundaries is reduced. As a result, continuous wave lasers are now suddenly attracting attention.
Trying to form a single crystal semiconductor film on an insulating surface is not new and a technique called graphoepitaxy has been devised as a more positive attempt. Graphoepitaxy is a technique in which a level difference is formed on a surface of a quartz substrate, an amorphous semiconductor film or a polycrystalline semiconductor film is formed on the substrate, and the film is heated by a laser beam or a heater so that an epitaxial growth layer is formed with the level difference on the quartz substrate as the nucleus. This technique is disclosed in, for example, Non-patent Literature 1.
[Non-Patent Literature 1]
J. Vac. Sci. Technol., “Grapho-epitaxy of silicon on fused silica using surface micropatterns and laser crystallization”, 16(6), 1979, pp. 1640-1643.
Another semiconductor film crystallizing technique called graphoepitaxy is disclosed in, for example, Non-patent Literature 2. The literature is about inducing epitaxial growth of a semiconductor film by artificially-created surface relief grating on an amorphous substrate. According to the graphoepitaxy technique disclosed in Non-patent Literature 2, a level difference is formed on a surface of an insulating film, a semiconductor film is formed on the insulating film, and the semiconductor film is subjected to heating, laser light irradiation, or the like to start epitaxial growth of crystals of the semiconductor film.
[Non-Patent Literature 2]
M. W. Geis, et al., “CRYSTALLINE SILICON ON INSULATORS BY GRAPHOEPITAXY”, Technical Digest of International Electron Devices Meeting, 1979, p. 210.
Crystalline semiconductor films formed using laser annealing methods, which are roughly classified into pulse oscillation and continuous wave, are masses of crystal grains in general. These crystal grains have varying sizes and are positioned at random, and it is difficult to specify the position and size of crystal grains in forming a crystalline semiconductor film. Therefore an active layer formed by patterning the crystalline semiconductor film into islands generally have interface between crystal grains (grain boundaries).
Unlike the inside of a crystal grain, a grain boundary has an infinite number of re-combination centers and trap centers due to an amorphous structure and crystal defects. When carriers are trapped in these trap centers, the potential of the grain boundary rises to block carriers and lower the current carrying characteristic of carriers. Therefore, grain boundaries in an active layer, in particular, in a channel formation region of a TFT, seriously affect TFT characteristics by lowering the mobility of the TFT greatly, by lowering ON current, and by increasing OFF current since a current flows in grain boundaries. Grain boundaries also cause fluctuation in characteristic among TFTs that are intended to have the same characteristic because the characteristic of a TFT having grain boundaries in its active layer is different from that of a TFT whose active layer has no grain boundaries.
Crystal grains obtained by irradiating a semiconductor film with laser light have varying sizes and are positioned randomly because of the following reason. It takes time for a liquefied semiconductor film that has been thoroughly melted by laser light irradiation to create a solid nucleus. As time passes, an infinite number of crystal nuclei are generated in the thoroughly melted region and crystals grow from the crystal nuclei. Since positions of the crystal nuclei to be generated are at random, they are distributed unevenly. Crystal growth is stopped as crystal grains collide against each other. Accordingly, the crystal grains obtained have varying sizes and are positioned at random.
Ideally, a channel formation region, which has a great influence over TFT characteristics, is formed from a single crystal grain removing adverse effect of grain boundaries. However, prior art is mostly unsuccessful in forming a crystalline silicon film with no grain boundaries by laser annealing. Therefore no TFT whose active layer is formed of a crystalline silicon film crystallized by laser annealing has succeeded in obtaining characteristics that rival the characteristics of a MOS transistor manufactured on a single crystal silicon substrate.